Descaling method for steel

ABSTRACT

Chromium-containing steel is descaled by wetting the surface of the steel after annealing and while still hot with a mixed aqueous solution of caustic soda and/or caustic potash containing at least one alkali metal salt capable of forming caustic soda or caustic potash by hydrolysis or with an aqueous solution of two or more alkali metal salts which are capable of forming caustic soda or caustic potash by hydrolysis. The water of the solution is evaporated by the retained heat of the steel and the mixture of alkali metal salts is retained on the surface of the steel in an oxygen-containing atmosphere for a period sufficient to convert the scale components into water- or acid-soluble materials.

iJnite Fukui et al. 1 Feb. 6, 1973 DESCALING METHOD FOR STEEL 1,049,054 7 1911 Coombs ..134/29 [75] inventors: Saburo Fukui; Tsunehiro Yamazaki, 25387O2 H1951 Noble: "134/29 X both of Hiroshima Masato 2,678,289 5/l954 Noblem. ..l34/3 Y A M 2,678,290 5/l954 Noble t ..l34/3 1 g i f 3,030,239 4/1962 Mekjean ..134/29 flab- 3,126,301 3/1964 Faler ..134 29 mo 0, nos 1ma, a o apan [73] Assignee: Mitsubishi Jukogyo Kabushiki pri'flary Exami'fer M orris wolk Kaisha, Tokyo Japan Assistant Examiner-Sidney Marantz [22] F] d 0 t 8 1970 Attorney-McGlew and Toren 1 e c 211 App]. No.: 79,290 ABSTRACT Related Application Data Chromium-containing steel is descaled by wetting the surface of the steel after annealmg and while still hot Continuation4WD?"t of S 10, April 21, with a mixed aqueous solution of caustic soda and/or 19691 abandoned and a commumlon'm'pan of caustic potash containing at least one alkali metal salt July 1969 capable of forming caustic soda or caustic potash by hydrolysis or with an aqueous solution of two or more [52] 3 22 alkali metal salts which are capable of forming caustic I t Cl 5 soda or caustic potash by hydrolysis. The water of the 4 E 15 solution is evaporated by the retained heat of the steel le 0 and the mixture of alkali metal salts is retained on the i surface of the steel in an oxygen-containing at- [56] References Cited mosphere for a period sufficient to convert the scale UNIT STATES PATENTS components into wateror acid-solublematerials.

3,467,549 9/1969 Bartek 1 34/3 X 20 Claims, N0 Drawings DESCALING METHOD FOR STEEL REFERENCE TO PRIOR APPLICATIONS This is a continuation-in-part of application Ser. No. 818,110, filed on Apr. 21, 1969 now abandoned and of application Ser. No. 838,904, filed on July 3, 1969 US. Pat.No.3,6l7,039.

FIELD OF INVENTION This invention generally relates to the descaling of steel and is particularly directed to an improved method of descaling chromium-containing steels such as chromium-containing special or alloy steels by treatment with fused alkali. The inventive procedure is particularly suitable for use in continuous processes wherein steel in band, strip or web formation travels through the plant and is descaled during its travel.

BACKGROUND INFORMATION AND PRIOR ART In recent years, the tendency has been to descale steels by a method which is generally referred to as fused alkali treatment according to which the undesired oxide coatings (hereinafter referred to as scale) on the exterior surfaces, which are formed inter alia during the rolling or annealing procedures, are removed for the purpose of improving the steel surface and with a view of meeting the ever increasing requirements, for example, increase of steel varieties and speed up of the descaling process line. According to the fused alkali treatment, only the scale and not the mother metal is attacked, an excellent finished surface is obtained, the processing requires a short time only and the descaling is completed by an after-treatment,

' wherein the steel body is immersed for a short period of time in an acid.

According to this prior art fused alkali treatment, the steel body, from which the scale is to be removed, is immersed in a molten bath of caustic soda or caustic potash mixed with additives. According to'the type of additives which are added to the caustic soda or potash, such prior art descaling procedures are referred to as the sodium hydride method or the Hooker method. In the sodium hydride method, a steel workpiece is immersed for a certain period of time in the caustic soda bath which is maintained at about 370 C. while 1.5 to 2 percent of sodium hydride is added as a reducing agent to the caustic soda. In this procedure, the scale component FeOCr O is converted into a readily acid-soluble iron or chromium oxide as indicated in the following equation 1 and is then subjected to washing with water or immersion in sulfuric acid, nitric acid or hydrofluoric acid for a short period oftime to complete the descaling procedure.

In more recent times, a further method has been proposed according to which the descaling is carried out while oxygen or air is bubbled through the molten bath of caustic soda or of caustic soda and sodium nitrate.

In all these prior art methods, an alkali metal compound is always employed in the form of a molten bath and a workpiece is immersed in the molten system. Such procedure is disadvantageous from a practical point of view, since it is cumbersome to prepare the molten alkali metal compound bath. This requires considerable expenditure in equipment and installation. Moreover, handling and operation are troublesome and connected with considerable hazards to the operator.

SUMMARY OF THE INVENTION vention to provide an alkali metal compound descaling I procedure which avoids the disadvantages and drawbacks of the prior art and wherein the steel can be effectively descaled without first having to prepare a molten bath of alkali metal compound.

Generally, it is an object of the invention to improve on steel descaling procedures as presently practiced.

Briefly, and in accordance with the invention, chromium-containing steels, such as chromium alloy steels, are descaled by first wetting the surface of the steel from which the scale is to be removed, directly after annealing, with a. a mixed aqueous solution of caustic soda and/or caustic potash containing at least one alkali metal salt capable of forming caustic soda or caustic potash, such as the carbonate, sulfate, silicate, nitrate and chlorate of sodium and potassium or an aqueous solution of two or more alkali metal salts which are capable of forming caustic soda or caustic potash by hydrolysis. The water of the solution evaporates instantaneously by the retained heat thus to form a thin film of fused salt. The fused state is maintained for a certain period of time in an atmosphere containing oxygen or in air, whereafter'the oxidation reaction represented by equation 3 is performed in gaseous phase.

It has been. ascertained that in accordance with the invention, effective descaling can be achieved in this manner without having to prepare a salt bath maintained at a constant temperature and without adding any oxidizing agent to the system. The inventive procedure may, however, be facilitated bythe use of an oxidizing agent such as nitrate or chlorate. This is so because the oxidation reaction then is synergistic, to

wit, the oxidizing agent acts from the inside and the oxygen in the ambient atmosphere acts from the outside. The descaling takes place more rapidly as a result.

In accordance with the method of the invention, the descaling can be effected even at a relatively high temperature, in particular, above about 600 C. without attacking the mother metal and affecting the finished surface. Moreover, the descaling can also be effected at a relatively low temperature. Therefore, the method of the invention is effective within a wide range of temperatures, for example, 250 to 950 C., the specific temperature depending on the steel varieties and line speed. The concentration of the aqueous solution is ordinarily to 50 percent by weight and that of caustic soda or caustic potash must be 1.25 percent or less. if the concentration is raised to higher values, the ground or mother metal is attacked by alkali in a high temperature treatment, resulting in finishing unevenness. The aqueous solution of alkali metal salts may be applied to the steel body in the form of a thin film by spraying. The amount of spraying and the concentration thereof are controlled depending on the steel variety as well as line speed.

The invention will now be described by several examples, it being understood, however, that these examples are given by way of illustration and not by way of limitation, and that many changes may be effected in the process conditions without affecting in any way the scope and spirit of the invention as recited in the appended claims.

EXAMPLE 1 A cold rolled steel plate, SUS 27, of 0.8 mm thickness was annealed at l,100 C., wetted with an aqueous solution of percent sodium carbonate 3 percent sodium nitrate 1 percent caustic soda, held in the air for 1-2 seconds so as to adjust the surface temperature of the steel plate to 800 600 C. and immediately cooled. The steel plate, after being washed with water, was in the state that the oxidation reaction of equation 3 was completed, followed by, as an aftertreatment, immersion in a mixed acid of 10 percent nitric acid and 2 percent hydrofluoric acid at 60 C. for 10 seconds to thus obtain a completely descaled, lustrous metal ground.

EXAMPLE 2 A cold rolled steel plate, SUS 24, of 0.5 mm thickness was annealed at 800 C., wetted with an aqueous solution of 5 percent sodium carbonate, 5 percent sodium sulfate and 0.5 percent caustic soda, held in the air for l-2 seconds so as to adjust the surface temperature of the steel plate to 600 300 C. and cooled. After being washed with water, the steel plate was substantially descaled and a somewhat cloudy metal ground was exposed, followed by, as an after treatment, immersion in an aqueous solution of 5 percent sulfuric acid at normal temperature for several seconds thus obtaining a completely descaled, lustrous metal ground.

EXAMPLE3 A similar procedure to that of Example 2 was repeated except that the potassium carbonate was replaced by sodium nitrate in the same amount. A similar effect could be obtained only by holding the steel plate for l-2 seconds in air so as to adjust the surface temperature to 600 400 C. and was not affected even on high temperature areas.

EXAMPLE 4 A cold rolled steel plate, A181 309, of 1.2 mm thickness was annealed at l,l00 C., wetted with an aqueous solution of 5 percent sodium carbonate 8 percent sodium nitrate 2.5 percent caustic soda, held in the air for about 10 seconds so as to adjust the surface temperature of the steel plate to 900 700 C. and then cooled. The steel plate, after being washed with water, was in the state that the oxidation reaction of equation 3 was completed, followed by, as an aftertreatment, immersing in a mixed acid of 15 percent nitric acid and 3 percent hydrofluoric acid at 60 C. for 20 seconds, to thus obtain a completely descaled, lustrous metal ground.

EXAMPLE 5 A cold rolled steel plate, SUS 27, of 0.8 mm thickness was annealed at 1,l00 C., wetted with an aqueous solution of 50 percent sodium silicate 3 percent sodium nitrate, held in the air for several seconds so as to adjust the surface temperature to 900 700 C. Then, the steel plate was washed with water and, as an after-treatment, subjected to immersion in a mixed acid of 15 percent nitric acid 3 percent hydrofluoric acid at normal temperature to thus obtain a completely descaled,-lustrous metal surface.

EXAMPLE 6 A cold rolled steel plate, SUS 24, of 0.5 mm thickness was annealed at 800 C., wetted with an aqueous solution of 5 percent sodium carbonate 1 percent sodium chlorate, held for l-2 seconds so as to adjust the surface temperature to 700 400 C. and then cooled. Thus, the steel plate was washed with water and immersed in 15 percent nitric acid at 60 C. for 5 seconds to thus obtain a completely descaled, lustrous metal surface.

As will be appreciated from the above, chromiumcontaining steels are descaled in accordance with the inventive procedure by wetting the surface of the steel from which the scale is to be removed, directly after annealing with a mixed aqueous solution of caustic soda and/or caustic potash containing one or'more of alkali metal salts which are capable of forming caustic soda or caustic potash by hydrolysis, such as the carbonate, sulfate, silicate, nitrate and chlorate of sodium and potassium or with an aqueous solution of two or more of alkali metal salts which are capable of forming caustic soda or caustic potash by hydrolysis, evaporating instantaneously the water by the retained heat thus to form a thin film of fused salt, holding the fused state for a certain period of time in air or in an oxygen-containing atmosphere to thus convert the scale on the surface of the chromium-containing steel into a wateror acid-soluble compound and then immersing in a usual inorganic acid such as nitric acid, hydrofluoric acid or sulfuricv acid.

The advantages and merits of the present procedure may be briefly summarized as follows, the summary considering particularly the prior art descaling methods whichinvolve immersion of the steel in fused alkali metal hydroxide baths:

l. A salt bath, which is difficult to handle, is not necessary and accordingly, the installation can be accomplished more economically.

2. Since descaling is carried out while utilizing th retained heat of chromium-containing steel after annealing, the operation cost is low.

3. Since the reaction ofa molten material in the form of a film with oxygen in the air is utilized skillfully, ad-

dition of an oxidizing agent is not necessary and the descaling speed is high.

4. No iron roll is necessary and, consequently, the finished surface has no scratches.

5. The handling of the descaling solution is very simple because the alkali metal salt is used in the form of an aqueous solution, not solidified at normal temperature.

6. Since the alkali metal salt is used in the form of an aqueous solution, the concentration can be suitably controlled according to the steel variety and scale nature and the quantity thereof carried away through adhesion on the surface of the steel is small.

7. Since the alkali metal salt is used in the form of an aqueous solution, selection of the quality of a spraying nozzle or apparatus is made easy, resulting in lowering of the installation cost.

8. Descaling can be accomplished more readily than in the prior art method using a fused salt, because the descaling treatment utilizing the retained heat on the steel and the fog quench can be simultaneously carried out in the process line and furthermore, the mechanical shock due to rapid evaporation of water on the surface aids stripping of the scale.

What is claimed is:

1. A method of descaling chromium-containing steel, which comprises 1. wetting the surface of the steel after annealing and while still in the hot state with an aqueous solution containing (i) a member selected from the group consisting of caustic soda, caustic potash, and mixtures thereof, and (ii) at least one member selected from the group consisting of alkali metal carbonate, alkali metal sulfate, and alkali metal silicate,

2. evaporating the water of the solution by the retained heat of the steel, and

3. retaining the mixture of alkali metal salts, molten at 250 950 C., on the surface of the steel in an oxygen-containing atmosphere for a period of time sufficient to convert the scale components into water-or acid-soluble materials, the concentration of said solution being from about 5 to about 50 percent by weight and the concentration of the caustic alkali not exceeding about 2.5 percent by weight of the solution.

2. A method as claimed in claim 1, wherein the solution consists of aqueous alkali metal carbonate, alkali metal sulfate and caustic soda.

3. A method as claimed in claim 1, wherein said alkali metal is selected from the group consisting of (i) sodium, and (ii) potassium.

A method as claimed in claim 1, wherein the concentration of the caustic alkali component (i) does not exceed about 1.25 percent by weight.

5. A method as claimed in claim 1, wherein the solution consists of aqueous sodium carbonate, sodium sulfate and caustic soda.

6. A method as claimed in claim 5, wherein the solution consists of 5 percent sodium carbonate, 5 percent sodium sulfate and 0.5 percent caustic soda, all of said percents being by weight.

7. A method of descaling chromium-containing steel,

which comprises 1. wetting the surface of the steel after annealing and while still in the hot state with an aqueous solution containing at least two members selected from the group consisting of alkali metal carbonate, alkali metal sulfate, alkali metal silicate, alkali metal nitrate, and alkali metal chlorate, evaporating the water of the solution by the retained heat of the steel, and 3. retaining the mixture of alkali metal salts, molten at 250 950 C., on the surface of the steel in an oxygenfcontaining atmosphere for a period of time sufficient to convert the scale components into water-or acid-soluble materials, the concentration of said solution being from about 5 to about 50 percent by weight and the concentration of the caustic alkali component (i) not exceeding about 2.5 percent by weight of the solution.

8. A method as claimed in claim 7, wherein the solution consists of alkali metal carbonate, alkali metal nitrate and caustic soda.

9. A method as claimed in claim 7, wherein said alkali metal is selected from the group consisting of (i) sodium, and (ii) potassium.

10. A method as claimed in claim 7, wherein the solution consists of aqueous potassium carbonate, sodium nitrate, and caustic soda.

11. A method as claimed in claim 10, wherein the solution consists of about 5 percent potassium carbonate, 5 percent sodium nitrate, and 0.5 percent caustic soda, all of said percents being by weight.

12. A method according to claim 7, wherein the solution consists of aqueous alkali metalsilicate and alkali metal nitrate.

13. A method according to claim 12, wherein the solution consists of aqueous sodium silicate and sodium nitrate.

14. A method according to claim 13, wherein the solution consists of about 50 percent sodium silicate and about 3 percent sodium nitrate, all of said percents being by weight.

15. A method according to claim 7, wherein the solution consists of aqueous alkali metal carbonate and alkali metal chlorate.

16. A method according to claim 15, wherein the solution consists of sodium carbonate and sodium chlorate.

17. A method according to claim 16, wherein the solution consists of about 5 percent sodium carbonate and l percent sodium chlorate, all of said percents being by weight.

18. A method as claimed in claim 7, wherein the solution consists of aqueous sodium carbonate, sodium nitrate and caustic soda.

19. A method as claimed in claim 18, wherein the solution consists of about 10 percent sodium carbonate, about 3 percent sodium nitrate, and about 1 percent caustic soda, all of said percents being by weight.

20. A method according to claim 18, wherein the solution consists of about 5 percent sodium carbonate, about 8 percent sodium nitrate and about 2.5 percent caustic soda. 

1. A method of descaling chromium-containing steel, which comprises
 1. wetting the surface of the steel after annealing and while still in the hot state with an aqueous solution containing (i) a member selected from the group consisting of caustic soda, caustic potash, and mixtures thereof, and (ii) at least one member selected from the group consisting of alkali metal carbonate, alkali metal sulfate, and alkali metal silicate,
 1. wetting the surface of the steel after annealing and while still in the hot state with an aqueous solution containing at least two members selected from the group consisting of alkali metal carbonate, alkali metal sulfate, alkali metal silicate, alkali metal nitrate, and alkali metal chlorate,
 2. evaporating the water of the solution by the retained heat of the steel, and
 2. A method as claimed in claim 1, wherein the solution consists of aqueous alkali metal carbonate, alkali metal sulfate and caustic soda.
 2. evaporating the water of the solution by the retained heat of the steel, and
 3. retaining the mixture of alkali metal salts, molten at 250* -950* C., on the surface of the steel in an oxygen-containing atmosphere for a period of time sufficient to convert the scale components into water-or acid-soluble materials, the concentration of said solution being from about 5 to about 50 percent by weight and the concentration of the caustic alkali not exceeding about 2.5 percent by weight of the solution.
 3. retaining the mixture of alkali metal salts, molten at 250* -950* C., on the surface of the steel in an oxygen-containing atmosphere for a period of time sufficient to convert the scale components into water-or acid-soluble materials, the concentration of said solution being from about 5 to about 50 percent by weight and the concentration of the caustic alkali component (i) not exceeding about 2.5 percent by weight of the solution.
 3. A method as claimed in claim 1, wherein said alkali metal is selected from the group consisting of (i) sodium, and (ii) potassium.
 4. A method as claimed in claim 1, wherein the concentration of the caustic alkali component (i) does not exceed about 1.25 percent by weight.
 5. A method as claimed in claim 1, wherein the solution consists of aqueous sodium carbonate, sodium sulfate and caustic soda.
 6. A method as claimed in claim 5, wherein the solution consists of 5 percent sodium carbonate, 5 percent sodium sulfate and 0.5 percent caustic soda, all of said percents being by weight.
 7. A method of descaling chromium-containing steel, which comprises
 8. A method as claimed in claim 7, wherein the solution consists of alkali metal carbonate, alkali metal nitrate and caustic soda.
 9. A method as claimed in claim 7, wherein said alkali metal is selected from the group consisting of (i) sodium, and (ii) potassium.
 10. A method as claimed in claim 7, wherein the solution consists of aqueous potassium carbonate, sodium nitrate, and caustic soda.
 11. A method as claimed in claim 10, wherein the solution consists of about 5 percent potassium carbonate, 5 percent sodium nitrate, and 0.5 percent caustic soda, all of said percents being by weight.
 12. A method according to claim 7, wherein the solution consists of aqueous alkali metal silicate and alkali metal nitrate.
 13. A method according to claim 12, wherein the solution consists of aqueous sodium silicate and sodium nitrate.
 14. A method according to claim 13, wherein the solution consists of about 50 percent sodium silicate and about 3 percent sodium nitrate, all of said percents being by weight.
 15. A method according to claim 7, wherein the solution consists of aqueous alkali metal carbonate and alkali metal chlorate.
 16. A method according to claim 15, wherein the solution consists of sodium carbonate and sodium chlorate.
 17. A method according to claim 16, wherein the solution consists of about 5 percent sodium carbonate and 1 percent sodium chlorate, all of said percents being by weight.
 18. A method as claimed in claim 7, wherein the solution consists of aqueous sodium carbonate, sodium nitrate and caustic soda.
 19. A method as claimed in claim 18, wherein the solution consists of about 10 percent sodium carbonate, about 3 percent sodium nitrate, and about 1 percent caustic soda, all of said percents being by weight. 